Rival, North and South Black Slough, Foothills, and Lignite fields in northeastern portions of the Williston Basin had produced 24.2 million barrels of oil through mid 1982 from the Mississippian Lower Charles Formation. These fields were discovered in the late 1950’s and unitized in the early to late 1960’s. In 1984 all, except Rival, were deunitized. The Rival subinterval at the base of the Charles Formation serves as the reservoir in Rival, North and South Black Slough, and Foothills fields. It is 30 to 60 feet thick. This subinterval was named at Rival Field but is commonly called the “Nesson”. The overlying Midale subinterval serves as the reservoir in Lignite Field and extends into southeastern Rival Field. It is 30 to 50 feet thick. Most oil production is from the Rival “Nesson” (21.3 MMSTBO, as of 1982). The study area is tilted basinward 2/3’s of one degree, with three subtle anticlinal noses trending through the fields.
The Rival “Nesson” represents a rapidly prograding shoreline and coastal sabkha which ceased building basinward and began slowly retreating. Barrier island and intertidal buildups of sparsely skeletal to skeletal, oolitic, pisolitic, intraclastic packstones formed along the shoreline. Most particles were micritized and neomorphosed to microspar. Inner portions of the shoreline were tightly cemented by anhydrite, while outer portions were partially cemented. Later, anhydrite was leached in outer portions of the shoreline to enhance reservoir porosity. Inner portions of the shoreline remained tight and along with anhydrite beds provide the updip stratigraphic trap. Some pores were later partially to completely filled with dolomite and calcite cement, drastically reducing permeability.
The Midale records a transgression which flooded the study area. Restricted marine to tidal flat, sparsely anhydritic, spiculitic, pelletal wackestone/packstones were dolomitized and serve as the reservoir. Porous dolostone is aphanocrystalline to very finely crystalline. Leaching of sponge spicule monaxons and some anhydrite further enhanced porosity.
The Rival “Nesson” pore system is composed of: (1) moldic and solution enlarged pores (10 to 1000 microns in width); (2) interparticle pores (5 to 25 microns); (3) intraparticle pores (5 to 10 microns); and (4) intercrystal pores (approximately 1 to 3 microns in width).
The Midale pore system is composed of: (1) moldic pores (5 to 500 microns in width); and (2) intercrystalline pores which are, (a) polyhedral pores (3 to 10 microns), (b) tetrahedral pores (3 microns), and (c) interboundary-sheet pores (1 micron in width).
